Chemical Reactions

Sarah Gleason

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Transcript of Chemical Reactions

When substances REACTLets take a look at what happens between some elements on the periodic table when they are combined Chemical Reactions EVERYTHING that you own is made from chemicals! Chemical Reactions And the atoms that make up those chemicals in your "stuff" are REARRANGED into new substancesRemember the Law of Conservation of Matter (Mass) CHEMICAL REACTIONS - the atoms from one or more substances are rearranged to form different substancesIt is another name for a "chemical change"They affect EVERY part of your life: break down your food when you eat, helps your cells produce energy, helps your engine work in your car, ect.... Signs to look for.... Did a Chemical Reaction Occurred? Sometimes its easier to tell,sometimes not, but here are a few signs:1. Temperature change2. Color Change3. Odor - new smell4. Gas Formation (like bubbles)5. Formation of a solid (or a change in state of matter) What we use to represent chemical reactions Chemical Equations They have a reactant and a productStarting substances (reactant) and ending substances (product) Arrow is known as "react" or "yields"NEVER equals because the reactant and product are NOT equal Coefficients Chemical Equations These are the large whole numbers written in front of a chemical formula to show you the amount of a compound or moleculeExample 3H2O - 3 water molecules Make sure that when you write out a chemical equation you ALWAYS write in the lowest ration possible for the reactants and products!! How to tell if a chemical equation is balanced. Is it balanced? Step 1: Write out the skeleton equation...if its not alreadyStep 2: Write out the elements that are listed in the equationStep 3: Count the atoms of each element - on the reactant side and the product side Step 1: Count the atoms in the chemical equation (previous steps) and proceed if unbalancedStep 2: Change the Coefficients ONLY to make the number of atoms in each element EQUAL! Remember that you can NEVER change the little subscripts, because it changes the identity of the substanceStep 3: Write the coefficients in their lowest ration possible (if they are already not)Step 4: Check you work - by recounting the atoms to make sure the it follows the Law of Conservation of Matter How to Balance Chemical Equations **The little numbers are called subscripts and do NOT ever change!! Balancing Example Fundamental concept of why we balance equations Law of Conservation of Matter Since matter is not created nor destroyed we must balance equations to demonstrate how the reactions would occur in real life situationsOtherwise we would be incorrectly demonstrating our chemical changes There are four types: some types fall in to more than one category: synthesis, combustion, decomposition, replacement Types of Chemical Reactions Start with Synthesis Reactions:Chemical Reaction where two substances combine to form a single product (A+B AB)Or when two compound combine to form a single compoundOr when a 1 compound combines with 1 element to form a single product (compound) When oxygen combines with a substance and releases energy in the form of light and heatAlso Known as BURNING Combustion Reactions Oxygen can combine with MANY different substances , which makes combustion reactions very commonSome Examples: Coal burned to produce energy and heatHydrocarbons - burn oxygen to yield carbon dioxide and water with coal and energy Combustion Coal is a fossil fuel that is mainly composed of carbon:Coal can form combustion reactions with oxygen that powers electric plants in many parts of the U.S. and the worldThat's why its the largest energy source in the world synthesis and combustion? Label the following Can an equation be both?? 2Na (s) + Cl2 (s) 2NaCl (s)C (s) + O2 (g) CO2 (g)2H2 (g) + O2 (g) 2H20 (g)CH4 + 2 O2 → CO2 + 2 H2O2 CH4 + 3 O2 → 2 CO + 4 H2O SynthesisBothBothCombustionCombustion Water and dinitrogen pentaoxide react to produce aqueous hydrogen nitrate. Both pretty much the opposite of synthesis reactions Decomposition Reaction A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide.These reactions are exothermic, meaning they produce heatThe release of heat can produce light in the form of either glowing or a flame. This is when a single compound breaks down into two or more elements/ new compoundsThey require an energy source to occur (like: heat, light, electricity, ect)AB → A + B One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:2 H2O ---> 2 H2 + O2 involve the replacement of an element in a compound Replacement Reactions First type is single-replacement reactions:A reaction where the atoms of one element replace the atoms of another element in a compoundA + BX AX + B When a metal replaces hydrogen (or another metal):Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)Or metals can replace another metal:Zn + CuCl2 ---> ZnCl2 + CuOr when a nonmetal replaces a nonmetal:Cl2 + 2NaBr → 2NaCl + Br2 Single - Replacement Metals differ in their reactivity, or ability to react with another substance:An activity series shows the order of metals by reactivityYou can use a reactivity series to predict whether or not a certain reaction will occurA metal can replace any metal in a compound if it is MORE REACTIVEIt CANNOT replace a metal if it is less reactiveThe letters NR are used to show that a reaction will not occur Predict if the reaction will occur or not, and if it will what the balanced product will be Single - Replacement Reactions Fe(s) + CuSO4(aq) ---> Br2(l) + MgCl2 (aq) --->Mg(s) + AlCl3 (aq) ---> FeSO4(aq) + Cu(s)NR (Br is below Mg)Al(s) + MgCl2(aq) Exchange of ions between two compoundsalso known as an exchange reaction or precipitation reaction Double - Replacement Reactions Follow this series of questions. When you can answer "yes" to a question, then stop!

1) Does your reaction have oxygen as one of it's reactants and carbon dioxide and water as products? If yes, then it's a combustion reaction2) Does your reaction have two (or more) chemicals combining to form one chemical? If yes, then it's a synthesis reaction3) Does your reaction have one large molecule falling apart to make several small ones? If yes, then it's a decomposition reaction4) Does your reaction have any molecules that contain only one element? If yes, then it's a single displacement reaction5) If you haven't answered "yes" to any of the questions above, then you've got a double displacement reaction This will hopefully help?! Exchange between positive charged ions (cations) and negatively charged ions (anions):AB + XY ---> AY + XBA and B represent the positively charged ionsX and Y represent the negatively charged ionsA positive ion always switches places to bond with negative ion Example: BaCl2(Aq) + Na2SO4(Aq) --> BaSO4(s) + 2NaCl(Aq)Notice that our positive ions are Sodium (Na) and Barium (Ba)Our negative ions are Chlorine (Cl) and Sulfate (SO4)The two positive cations have switched places with the two negative anions All double replacement reactions:Produce either water, a precipitate or a gas as a productA precipitate is a solid produced during a chemical reaction in a solution - when a solid is produced in a chemical reaction in a liquid Practice List what type the following reactions are: